Thermal ink-jet printers employ a plurality of resistors to fire droplets of ink through nozzles toward a medium, such as paper, in a pattern to form alphanumeric characters. It is desirable to use plain paper, rather than specially coated paper, as the print medium. However the matching of ink to paper is not easy, and many considerations, such as properties of the ink, reaction of the ink composition with the thermal ink-jet printhead, and interaction of the ink with the print medium, must be addressed.
An ink that gives good print quality on paper must also perform well in the printhead. As used herein, print quality is measured in terms of small dot size for a given drop volume. A 100 picoliter drop volume is typically used as a convenient reference.
An ink that performs well evidences low spray, good directionality as ejected from the nozzle, and fast firing frequency. By low spray is meant that the area around the perimeter of a printed character should be clear of stray droplets. By good directionality is meant that the angular deviation of an ejected droplet of ink from a nozzle should be within about .+-.0.5.degree. from the normal to the plane of the nozzle. By fast firing frequency is meant a firing rate of at least about 4 to 5 kHz. Such higher firing rates permit faster printing.
There are presently several thermal ink-jet printers that operate in the range of about 2.6 to 3.6 kHz. An ink composition presently used with such printers comprises a vehicle of about 95% water and about 5% diethylene glycol (DEG) and a dye of about 2.5% (all amounts in percent are by weight, unless otherwise indicated). Other additives, such as biocides and pH adjusters, may also be present.
While such low DEG (&lt;10%) inks are suitable for the purpose intended and evidence good print quality (e.g., low feathering and low bleed) on plain paper, there remain severe problems with spray, directionality, and crusting of the nozzles by the ink. Further, this ink composition is limited to firing frequencies below about 3 kHz.
There are other printers, using special papers, that are characterized by good print quality on such special papers. These printers employ a high DEG (50 to 70%) ink. The print quality of such inks on plain paper is degraded compared with that on the special papers, and such inks evidence slow drying on plain paper. On the other hand, crusting is relieved by the use of the high DEG ink, and it has been discovered that these higher viscosity inks are capable of firing at the higher frequencies mentioned above.
Thus, there remains a need to provide an ink composition having the best properties of both compositions: the good dry time and print quality on plain paper, together with the desire to print on plain paper at the higher frequencies.